High performance liquid chromatography is a well known method of separating solute species in dependence upon the differential absorption/desorption between different solute species. Typically, a liquid carrier (in which the solute species to be separated are present) is passed through a column packed with separation media (e.g., solid or gel particles). This separation media, in effect, increases the residence time of one (or more) solute species in the liquid carrier (which is inert to the solute species) relative to one (or more) other solute species in the liquid carrier (i.e., due to the greater rate of absorption/desorption of the one solute species relative to the other solute species). Due to the increased residence time of the one solute species in the column, there will be a time when an essentially pure mixture of carrier liquid and the other solute species will be present at the discharge of the column--that is, the one and other solute species will b separated.
With the recent advent of commercial manufacture of biological species (e.g., proteins), however, the conventional liquid chromatography technique of using packed particle beds has proven to be an inefficient means of separating one species from another with high resolution. This inability of packed particle chromatography columns can be attributed generally to the high pressure drops which are experienced and which lead to lesser flow rates of the liquid carrier through the column (thereby leading to lesser rate of production of the desired biological species). In addition, the very stringent control over particle size, the uniformity of the particles and the manner in which such particles fill the column contribute to increased costs which, in general, cannot be tolerated on a commercial scale. Hence, the conventional packed particle technique for liquid chromatography, while being adequate for analytical purposes on a small scale, is inadequate for separating solute species (particularly biological species) on a commercial scale.
Recently, it has been proposed to employ hollow fibers for liquid chromatography. The geometry of such hollow fibers provides an attractive alternative to particles in terms of lesser pressure drop through the column, and the lower cost of hollow fibers (relative to uniform particles) which provides attractive economies of scale and thus may allow commercially viable liquid chromatography systems to be achieved.
One prior proposal in this regard employs a tube packed to between 60 to 100 percent of the tube's theoretical packing density with generally parallel aligned fibers so as to provide the means to perform liquid chromatography. See, U.S. Pat. No. 4,657,742 issued on Apr. 14, 1987 to Richard P. Beaver. The fibers in U.S. Pat. No. 4,657,742 are preferably glass fibers which may be solid or they may be solid fibers that have been rendered porous. Alternately, the fibers may be hollow fibers having solid walls or hollow fibers whose walls have been treated so as to render them porous. The process for packing the tube with a high density of fibers is also said to be applicable to fibers other than glass, for example, cellulosic and organic fibers (see column 10, lines 10-13 of USP 4,657,742).
In Japanese Publication No. 55-18244, there is disclosed a hollow fiber which is suitable for use in liquid chromatography. The hollow fiber disclosed therein includes an external liquid-impermeable skin layer and an internal (i.e., adjacent the lumen of the fiber) porous layer. The liquid-impermeable layer apparently allows liquid chromatography to proceed with increased pressure within the internal lumen of the fiber without carrier liquid "breakthrough" to the external side of the fiber.
While the above-mentioned proposals for liquid chromatography may provide some improvement over the conventional packed particle columns used in liquid chromatography applications, there still exists a need in this art for greater solute separation resolution on a commercially viable scale--that is a liquid chromatography system which is adapted to separate a greater amount of solute species per unit time as compared to conventional liquid chromatography techniques. It is towards achieving such needed improvements that the present invention is directed.
According to the present invention, microporous hollow fibers (preferably polyolefinic fibers which are normally hydrophobic) are employed as the means to immobilize, within the micropores of the fiber, a stationary phase having a greater affinity to at least temporarily absorb at least one solute species relative to at least one other solute species. Thus, when a liquid carrier containing the solute species to be separated is passed through the central lumen of the hollow fiber, at least one of the solute species will have a greater residence time within the hollow fiber (i.e., due to its affinity to be at least temporarily absorbed by the immobilized phase within the fiber's micropores), thereby causing the solute species to be separated at the outlet end of the fiber--that is, the solute species with which the immobilized phase has a lesser affinity for absorption will appear in essentially "pure" form (but carried by the carrier liquid) before the solute species with which the immobilized phase has a greater absorption affinity.
Preferably, the immobilized phase within the fiber's micropores is organic since it more easily "wets" the normally hydrophobic hollow fibers (i.e., so that the organic phase fills completely the micropores of the hollow fiber). The immobilized phase may thus be an organic liquid or a gel, with the particular chemical composition of the immobilized phase being selected in dependence upon a number of factors, including its compatibility with the material of the hollow fiber and/or its compatibility with the solutes to be separated, and/or its absorption/desorption affinity for particular solute species of interest to be separated.
Therefore, according to the present invention, a commercially viable alternative to prior liquid chromatography techniques is provided. This advantage, as well as others, will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.